The present invention relates to pumpable heat transfer compositions and to the method of making the same.
It is known to utilize heat transfer compositions for a wide variety of purposes, such as coolants, heat sources, increased efficiency from constant temperature fluids, and reduction of peak load energy requirements by storage potential, as well as in other areas in order to increase the potential benefits from solar energy and waste heat collection. There are a number of materials that have been utilized for this purpose with those having the highest heat capacities and heat transfer coefficients being those most suitable for this purpose. Examples of such known heat transfer materials are water, certain glycols, hydrocarbons, fluorinated hydrocarbons, and the like. For certain uses, such as military applications, where it is desired to use dielectric coolants in aircraft radar and missile systems and other avionic components, it has long been known to utilize silicate-ester based coolants and more recently the readily available hydrogenated polyalphaolefin (PAO) based fluids. However, even the PAO's do not have a satisfactorily high heat capacity and heat transfer coefficient. Attempts to add a phase change material (PCM) thereto in order to increase the heat capacity and heat transfer coefficient of the PAO's have not been entirely successful.
Also, at the present time hydrophilic silicates are utilized as the solid microporous structure to carry the PCM's. If silicates do break during pumping, they will still absorb the PCM and would continue to function.
One cannot utilize any PCM soluble in the PAO, such as a paraffin, since it will solidify the entire composition and the use of an insoluble PCM will solidify in the heat rejector and plug up the flow lines.
Efforts to overcome some of these problems have included encapsulating the PCM's. The phase change of interest is a liquid to solid and vice versa with heat transfer equivalent to the latent heat of solidification (preferably crystallization). Use of PCM's, soluble in the PAO, is not satisfactory since the PCM and PAO being soluble in each other diffuse through the capsule shell and one then has the same problems as before. Capsules also have a tendency to rupture due to, at least, thermal cycling and pumping. The resultant capsule breakage and the loss of the PCM will degrade the thermal properties of the heat transfer composition, thus, vitiating the beneficial effects of the PCM. Thus, even if an insoluble PCM is utilized, capsule breakage will also eliminate its beneficial effects.
For other heat transfer applications, microcapsules containing PCM are impractical because of eventual breakage through pumps and flow channels. For example, this is known for water-based and glycol-based slurries containing paraffin PCM's within nylon-coated microcapsules. This was demonstrated by the U.S. Army in cooling vest applications.